Why won't my G string stay in tune?
If your guitar has a single string that seems to go out of tune but the other strings hold their tuning OK, there is one common likely reason. Assuming that you gave checked that the string is not slipping on the tuning peg (which usually gives a big drop in pitch), then the most likely problem is that the string is sticking in the guitar's nut.
Your guitar's nut has a slot cut in it for each string and these slots obviously vary in width to cope with the different size of each string. If the strings you have fitted are slightly thicker (heaver gauge) than were fitted to the guitar from new, then the string is held in the slot and cannot move freely as you stratch the string when tuning. The tension in the string is not even, and often once you start playing your guitar the string tension will even out, leading to an out-of tune string. This problem is particularly common on the G string as guitarists have the choice of a plain or wound string.
So, what can I do to fix it?
The good news is that this is an easy problem to fix. there are several tricks to this, listed in order of ease...
1) Loosen the string in question and apply some pencil lead to the string at the point where it passes through the nut. The graphite in the pencil will lubricate the slot and allow the string to move more easily when tuning. A soft pencil (B) is best, but almost any pencil will do. Be carefull not to overdo it and make a mess all over your guitar's neck... be sure to wipe of the surplus graphite dust.
2) As 1), but use a proprietry product such as "Big Bends Nut Sauce". http://www.bigbends.com/ This is particularly good if you use a lot of string bending in your guitar playing style. It basically does the same as the pencil lead (graphite), but better and cleaner.
3) Very carefully file out the slot in the nut to allow the string to move easily. This requires a great deal of care and, although not difficult if you have the correct needle files, is probably best left to a guitar technician or luthier - Ask at your local guitar shop; they should be able to do it for very little cost. This will also need doing if you have put significanly heavier strings on your guitar.
That's it - job done! You should now have a G string that you can tune and stays in tune.